Over the last twenty years, notable advancements to the science and technology of organic second-order nonlinear optical (NLO) materials have been made [see for example: L. R. Dalton, P. A. Sullivan, D. H. Bale, Chem. Rev. 2010, 110, 23-55]. Organic NLO materials are especially useful in electro-optic (EO) modulators as the active optical waveguide component [see for example: “Hybrid cross-linkable polymer/sol-gel waveguide modulators with 0.65 V half wave voltage at 1550 nm,” Y. Enami, D. Mathine, C. T. DeRose, and R. A. Norwood, J. Luo, A. K.- Y. Jen, N. Peyghambarian, Optical Materials 2011, 33, 1307-1315]. EO modulators are used, for example, to transduce information from an electrical signal to an optical signal that can travel, for example, on optical fibers. EO modulators change (modulate) the phase and amplitude of an optical carrier signal by means of changing (modulating) the index of refraction of the optical waveguide material. The transduction of information is accomplished by applying the voltage of an electrical signal (carrying the data to be transduced to the optical signal) across the active optical waveguide. The index of refraction of the NLO material (also called the EO material) is changed in proportion to the intensity of the applied electric field, thereby changing the phase of the optical signal and also making possible the construction of an optical amplitude modulator based on interferometry. The index of refraction change of the EO material is based on the second-order nonlinear optical response (also called a three-wave mixing response), and involves only electronic motions in the dye. Thus, a distinguishing property of the organic three-wave mixing NLO materials is that the frequency of modulating (or speed of changing) its index of refraction may exceed 100 gigahertz (or be less than 0.1 nanosecond).
The electro-optic (EO) coefficient (r33) is a well-known and useful figure-of-merit for comparing various NLO materials for use in EO modulators. The nomenclature “dye” and “chromophore” are used herein interchangeably. The dye is the key active component of the EO material. The dye has a long π-electron-conjugated backbone (also called the electronic conduit, and also called the charge-transfer conduit) that normally runs along the longest geometric axis of the dye. Dyes used in second-order NLO materials must be asymmetric, and they typically have an electron donor group on one end of the dye and an electron acceptor group on the other end of the dye. The magnitude of r33 for films of these of high-bandwidth organic-dye-based EO materials is largely determined by the product of three molecular parameters (see Equation 1):r33∝βNd<cos3(θ)>  Eq. (1)where β is the first molecular hyperpolarizability of the dye; Nd is the molar concentration of dyes (the number density), and <cos3(θ)> is the polar order parameter (polar alignment) of the ensemble of dyes.
One common method to increase the polar order of the dyes in a film is to apply a large voltage across a film that has been heated near or above its glass transition temperature (Tg). Under those conditions, the ground-state dipole moments of the dyes align with the applied poling field, then the film is cooled to ambient (well below Tg) before the poling field is removed, which freezes in the polar order. The film can also be crosslinked during poling to freeze in the polar order.
The method of Density Functional Theory (DFT) is a practical desk-top computational tool for predicting the relative changes in β as a function of specific changes in the chemical structure of the dye molecule. DFT calculations (theoretical calculations) of β can be performed using the Gaussian03 program as described in reference [Optical Materials 2011, 33, 1307-1315]. DFT calculations also predict the dipole moment of the dye (μ). The dipole moment of the dye (μ) is also a useful quantity for comparing dyes, because very large dipole moments can be detrimental if the dipole-dipole repulsion between neighboring dyes lowers the overall polar alignment. This normally occurs when the dyes are packed closely together at high concentrations, for example, at Nd>1019 dye molecules per cubic centimeter.
A distinguishing chemical structure of the here-to-for best-performing type of dye used in EO modulators is the phenylene-tetraenic π-electron-conjugated backbone. Each “ene” group in the polyene backbone of the dye includes two methine atoms linked by a double bond (the methine atoms are sp2 hybridized). An ene unit is also called a vinylene unit. One type of here-to-for best-performing dye includes the 4-aminophenyl-tetraene-tricyanofuran molecular framework, in which an isophorone unit is used to rigidify (or ring-lock) the tetraenic backbone. This type of dye is called the “phenylene-tetraenic” dye or “phenyl-tetraenic” dye herein. Examples of early publication are: U.S. Pat. No. 6,348,992 and Cheng Zhang and Larry R. Dalton, Min-Cheol Oh, Hua Zhang, and William H. Steier, Chem. Mater. 2001, 13, 3043-3050.
Another type of phenyl-tetraenic dye includes the 4-aminophenyl-(2,5-divinylene-heterocyclodiene)-tricyanofuran molecular framework. For example, see: X. Ma, F. Ma, Z. Zhao, N. Song, J. Zhang, J. Mater. Chem. 2010, 20, 2369-2380. These dyes are also considered here-to-fore among the best performing dyes for use in electro-optic modulators. Examples of heterocyclodienes are thiophene, pyrrole, thiazole and similar heterocyclic rings that include two ene groups. Thus, the 2,5-divinylene-thiophene is a tetraenic backbone unit.
A known strategy for increasing the β of organic dyes is to increase the length of the π-electron conjugated backbone (the electronic conduit) of the dye by adding another ene unit, for example, making the dye a pentaenic dye. However, increasing the length of the π-electron conjugated conduit of the well-known phenylene-tetraenic dyes has invariably resulted in dyes that exhibit increased and unacceptable optical absorption in the ˜1550 nm device operating window. This is because adding one more ene group extends the low-energy side of the electronic absorption envelop (the red tail). The problem arises when the red-tail wavelengths extends into and overlap with the 1550 nm optical carrier signal, which leads to unacceptable optical absorption loss. For the Mach-Zehnder modulator, the typical upper limit for acceptable optical propagation loss is ˜0.25 dB/mm, which also sets the upper limit on concentration of the dye in the film (and thus the upper limit on r33), which is a well-known trade-off. Furthermore, red-shifting of the tail of the electronic absorption spectrum to wavelengths greater than 1550 nm increases the rate of detrimental photo-chemical reactions. The near-infrared (NIR) attenuation caused by the dyes in an optical waveguide (at least for relative comparisons) can be estimated by measuring the optical absorption of a solution of known concentration of dye as a function of optical wavelength using a conventional ultraviolet-visible (UV-Vis) spectrometer.
Another known strategy for increasing the β of organic dyes is the incorporation of strong (relative to hydrogen) electron-withdrawing and strong (relative to hydrogen) electron-donating groups on the internal methines of large dyes. Methines are the sp2 carbons, and other hetero sp2 atoms, that make up the π-conjugated conduit of the dye. DFT computations have shown that β is usually enhanced when electron-withdrawing groups (W) are attached to even-numbered (e) internal methine carbons and electron-donating groups (D) are attached to odd-numbered (o) internal methines, the so-called eW/oD pattern. The methine numbering system for the dyes used herein is the same numbering system used in this reference: A. P. Chafin, G. A. Lindsay, J. Phys. Chem. C 2008, 112, 7829-7835.
Bulky, steric-hindering groups attached to the dye are needed to prevent aggregation and crystallization of large dyes at high loadings (at high concentrations). Attachment of large groups, such as t-butyldiphenylsiloxy, diphenylmethylsiloxy and similar-sized groups, to the ends and mid-section of the dye is a well-known method for improving solubility of the dyes. Attaching to the dye several linear or branched alkyl groups that include two to eight carbon atoms is also a well-known method for improving solubility of dyes. Such large groups are considered to be one type of “functionalized group,” a terminology used herein.
It is to be understood that the foregoing is exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.